I'm making a bunch of 2D games (now and in the near future) that use a 2D, RPG-like interface. I would like to be able to quickly paint tiles down and drop character sprites to create concept art.

Sure, I could do it in GIMP or Photoshop. But that would require manually adding each tile, layering on more tiles, cutting and pasting particular character sprites, etc. and I really don't need that level of granularity; I need a quick and fast way to churn out concept art.

Is there a tool that I can use for this? Perhaps some sort of 2D tile editor which lets me draw sprites and tiles given that I can provide the graphics files.